1. Field of the Invention
The present invention relates generally to a printing head or image transfer head for printing or transferring images such as characters and pictures, and more particularly to a high-speed, high-quality thermally recording or printing head which is adapted to locally energize an electrically resistive layer provided on an ink film or ribbon or on a recording medium, in order to generate Joule heat. The heat generated in the electrically resistive layer can be used for "thermal-printing" such as thermal-transfer printing or printing on a thermosensible paper or medium. Since a typical application of the above-mentioned printing head is found in the thermal-transfer printing using a film or ribbon having an electrically resistive layer, the present invention will be described mainly in connection with the thermal-transfer printing. However, it is to be understood that the principle of the invention is applicable to the other types of thermal printing such as printing using a thermosensible paper or medium.
2. Discussion of the Prior Art
Various thermal printing or image transfer heads operable with such a thermally fusible and transferable ink material are known. For example, such thermal printing heads are disclosed in Japanese Patent Applications which were laid open in 1985 as Laid-open Publications 60-214973, 60-214972, 60-214971 and 60-199669. As described in these publications, the printing of images according to the disclosed thermal image transfer method is effected by using an electrically resistive layer of a film, ribbon or a recording medium, and an ink layer consisting of a thermally fusible or vaporizable ink material. The electrically resistive layer is locally energized by an electric current applied thereto by recording electrodes of a printing head, so that the energized portions of the electrically resistive layer generate Joule heat, and thereby soften or vaporize an ink material on the corresponding portions of the ink layer. The softened or vaporized ink material is transferred to the surface of a recording medium, whereby an image corresponding to the softened or vaporized portions of the ink layer is recorded on the medium. In this type of thermal printing system, the recording electrodes of the printing head must be held in contact with the electrically resistive layer of the film, ribbon or recording medium, and are subject to wear due to frictional contact with the electrically resistive layer. With this operating condition taken into account, the recording electrodes of the printing heads proposed in the above-identified documents are made of tungsten, molybdenum, or other metals which have a high degree of wear resistance.
However, extensive studies of such printing heads revealed progressive deterioration in the wear resistance of the recording electrodes made of such electrically conductive materials, during a long period of use. Further the studies indicated rapid consumption of the positive side high-potential electrodes or anodes, which may develop into problems such as insufficient electrical contact of the electrodes with the electrically resistive layer, inconsistent contact pressure between these two members, and consequent deterioration of quality of the images to be printed on the recording medium.